This invention relates to a voltage controlled oscillator system. It has particular utility in a magnetic flowmeter system and will be described as applied to such a system, but its utility is not limited thereto. The invention also relates to a digitally calibrated flowmeter system which may embody the voltage controlled oscillator system.
Fluid flowmeters produce an output signal which is indicative of flow rate. In the case of a magnetic flowmeter, a magnetic field across a flow tube generates in the fluid a voltage which is proportional to the flow rate. This voltage is sensed by a pair of electrodes in contact with the fluid and amplified by a signal processing system. The output of the processing system is generally in the form of a DC (analog) signal, the magnitude (voltage) of which is proportional to flow rate.
In presently known magnetic flowmeters, a number of potentiometer adjustments must be made in order to calibrate the meter properly and to set the zero point accurately. These adjustments must be made by trained technicians, and once made are nonetheless susceptible to vibration and humidity. In presently known meters, the number of adjustments necessary has made digital calibration impractical. The number of adjustments is not made necessary by the flow signal, which varies in a highly linear manner with flow rate, but by the offset and non-linear response of the signal processing circuitry.
In certain magnetic flowmeters, the analog signal output is converted to a digital signal, such as a train of pulses, the frequency or duty cycle of which is proportional to flow rate. For such flowmeters, a voltage to frequency converter such as a voltage controlled oscillator would be a particularly convenient means of producing the output signals. Unfortunately, conventional voltage to frequency converters have a temperature coefficient of fifty parts per million per degree Celsius or more, which makes them unacceptable for high precision systems.